In our patent specification EP-A-0542779 we describe a pipe coupling for connecting together two plain-ended pipes in a fluid-tight manner, of the type consisting of a tubular casing formed with a longitudinal gap, a sealing gasket of resilient flexible material typically of rubber or synthetic rubber, arranged within the casing, and tensioning means for reducing the width of the longitudinal gap so as to tighten the casing around the gasket. In use, the sealing gasket is placed around the adjacent pipe ends and the tensioning means are tightened to clamp the gasket against the outer surfaces of the pipe ends to form a fluid-tight seal.
Fluid pressure in a pipeline tends to force the pipe ends apart. The coupling of the above-mentioned patent specification has an arrangement for securing the pipe against axial movement relative to the coupling under such loads. The sealing gasket has circumferential slots in its outer surface at each end of the gasket. A frusto-conical gripping ring with inwardly projecting gripping teeth is located in each slot. In use, when the tensioning means are tightened around the casing the gripping teeth penetrate the sealing gasket at the bottom of the slot and engage the outer surface of the pipe to hold the pipe ends against axial displacement.
The known anchoring arrangement is satisfactory for many applications. We have found however that there are circumstances where an anchoring arrangement is required that will provide a better grip, for example, for pipes made of hard material, such as some grades of stainless steel, for example 316Ti, pipes that are not truly circular in cross section due to variations in manufacturing tolerances, and pipes that have to carry fluid at very high pressure. In some marine applications, for example, pipes are required to operate at 16 bar pressure. Safety regulations require couplings to perform at four times the operating pressure, that is 64 bar. The axial load on a coupling at 64 bar may be sufficient to cause the teeth to slip on the surface of the pipe in some situations.
With the known coupling, under normal operation, as the axial load on the pipe increases, the frusto-conical gripping ring tends to flatten, causing the teeth to bite deeper into the pipe surface, improving the grip. However, when the pipe is of a particularly hard material or misshapen, under large axial loads such as would be produced at 64 bar, slippage may occur due to a tendency for the teeth to ‘jump’ on the surface of the pipe. We have found that this ‘jumping’ occurs because the gripping ring does not engage the surface of the pipe uniformly around its circumference. This variation in grip arises due to the inability of the gripping ring to conform to variations in the shape of the pipe and variations in the load applied at different points around the circumference by the casing due to the presence of the longitudinal gap and the increased stiffness in the region where the tensioning means are attached to the casing which may result in the casing tending to flatten in some areas.
The gripping ring of the known coupling is formed of a single strip of steel pressed into conical form and wound into a helix so that when in place in the coupling the strip subtends an angle of more than 360° at its centre of curvature and the ends of the strip overlap to form a complete ring.